1. Field of the Invention
The invention relates generally to energy (shock) absorbing devices, and more particularly to a friction dampener adapted to railway vehicle motion control.
2. Description of the Prior Art
Various types of shock absorption and dampening devices have been employed for many years in a variety of different applications. The most prevalent of such devices is probably the shock absorber, which is frequently used in a variety of different kinds of vehicles. The most common type of shock absorber is probably the hydraulic shock absorber, which utilizes a fluid filled cylinder and a plunger mechanism. Typically, the cylinder housing and the plunger are separately connectable to external elements which are expected to undergo relative displacements. However, there can be many applications where an energy absorber is called for, but where the hydraulic shock absorber is not necessarily desirable for many possible reasons. For example, modem hydraulic shock absorbers are relatively complex, expensive, and can be much heavier than function type dampening devices. Furthermore, because of the fluid medium utilized in hydraulic shock absorbers, the orientation of such shock absorbers can have an effect on performance. For example, hydraulic shock absorbers are generally more efficient when mounted in a relatively vertical orientation. Yet, there can be many applications where it is desirable to mount a shock absorber in a horizontal orientation. In particular, one such application is where a shock absorber, or dampener, is utilized as a yaw dampener on a railway vehicle truck assembly to control hunting.
Friction type shock absorbing devices, such as a friction dampener, can be simpler in design, less expensive, lighter weight, and unaffected by the orientation in which it is mounted. Various types of friction dampeners have been employed in the past to provide shock absorption or cushioning features where the use of hydraulic shock absorbers was undesirable. Prior friction dampeners can be one or two way energy absorbers and commonly comprise a first member, such as a cylindrical housing, a second member such as a shaft or rod, which is coaxially disposed within the housing, and a friction pad assembly carried by the shaft in sliding engagement with the inside surface of the housing. Both the housing and the shaft or rod are provided with a connecting member for attachment to external elements which are expected to undergo relative displacement. Such a friction pad assembly generally includes a friction element and some type of wedge member to initially set (or reset) the pressure between the friction element and the inside surface of the housing. Two examples of such friction dampeners are disclosed in U.S. Pat. No. 3,866, 67,724 and U.S. Pat. No. 3,796,288, both to Holnick. The friction dampener disclosed in both of the aforementioned patents provide a manual adjustment means for moving a wedge shaped member between, or out from between, the friction element in order to increase or decrease the force applied to the friction element against the inside surface of the housing.
Other friction dampeners have provided a leaf spring to bias the friction element against the housing. One such device is disclosed in U.S. Pat. No. 3,121,218 to Hallinan. Such a device can have a leaf spring that is bimetallic and manually adjustable to either urge the friction element against the housing to increase the frictional engagement, or reduce such pressure to decrease friction.
Still other friction dampeners have provided for a small servo motor to automatically adjust the wedge member to increase or decrease the pressure between the friction element and the inside surface of the housing. One such device disclosed in U.S. Pat. No. 5,080,204 to Bower et al. discloses a friction dampener for the drum of a washing machine unit. In Bower, a small servo motor is provided inside the dampener to operate a piston which moves the wedge member in order to decrease the pressure between the friction element and the inside surface of the housing. The servo motor is a thermoactuator element that is responsive to a rotational speed sensor which triggers the servo motor when a predetermined rotational speed has been exceeded.
However, such friction dampener devices can be associated with certain disadvantages resulting from the nature of the friction element when such devices are employed in some heavy load applications. An example of such an application is the use of a friction dampener on a railway vehicle truck assembly to control hunting, as referred to previously, wherein the truck assemblies can be carrying hundreds of tons of materials. During operation of a friction dampener in such an application, the extremely large forces which the dampener must control can result in very high temperatures being generated by the frictional interaction between the housing and the friction element. Because the friction element typically expands in response to an increase in temperature, this can causes a corresponding increase in the peripheral pressure on the housing. As might be expected, this increase in peripheral pressure normally results in causing the dampener to become increasingly stiffer. Compounding the situation further, the friction coefficient of the friction element is typically sensitive to temperature. Thus, as the temperature increases the friction coefficient of the friction element usually also increases. This is analogous to the situation where race car drivers spin the tires on the car to get the tire temperature up so they stick to the track better and resist slipping. A rubber/elastomeric compound experiencing this condition is commonly referred to as being "tacky." Thus, like the race car tires, the friction element can resist sliding on the inside surface of the housing as the temperature increases. The result can be that during operation of the friction dampener the pressure becomes so great and the friction element becomes so tacky that essentially no relative movement can occur between the friction element and the housing in the normal operating range. Basically the friction dampener can "lock up," at which point the dampener begins to behave like a fixed rod. This very undesirable condition can result in damage to the friction dampener or the externally connected elements which are expected to be able to move relative to each other. Furthermore, since the yaw forces are not being dampened, hunting of the truck assembly can get out of control.
Therefore, friction dampeners which do not provide some means for controlling the peripheral pressure between the friction element and the housing can be unacceptable in certain applications where the friction dampener must control heavy loads and undergo large variations in temperature and pressures. Accordingly, there is a need for a friction dampener which can generally maintain the peripheral pressure between the friction element and the housing within a preferred range of acceptable operating pressures.